1. Field of the Invention
The present invention relates to an exhaust gas control apparatus for an internal combustion engine and particularly to an exhaust gas control apparatus for an internal combustion engine that includes a nitrogen oxide (NOx) purification catalyst and an oxidation catalyst.
2. Description of Related Art
A technology for purifying exhaust gas from an internal combustion engine such as a gasoline engine or a diesel engine has been known that uses a NOx purification catalyst. However, use of noble metals in general used as catalyst components of the NOx purification catalyst, for example, platinum group metals such as rhodium (Rh) has been increasing accompanying reinforcement of regulation of automobile exhaust gas. Therefore, there has been a concern about the depletion of the resources. Accordingly, it is required that the amount of the platinum group metals used be reduced and such a function of the platinum group metals be provided by other metals instead.
Japanese Patent Application No. 2008-309013 (JP 2008-309013 A) discloses an exhaust gas control apparatus for an internal combustion engine that includes a fundamental structure having a first stage base metal catalyst which partially oxidizes mainly hydrocarbons (HC) and produces carbon monoxide (CO) and a second stage base metal catalyst which reduces and purifies NOx. Further, a preferable mode is disclosed such that an exhaust gas at an air-fuel ratio which is slightly more fuel rich than the stoichiometric air-fuel ratio is caused to flow into the first stage base metal catalyst, a third stage base metal catalyst which oxidizes and purifies HC and CO is provided on a downstream side of the fundamental structure, and air is introduced into the exhaust gas flowing into the third base metal catalyst by an air introduction apparatus provided between the fundamental structure and the third base metal catalyst. JP 2008-309013 A describes that iron (Fe) can be used as a catalyst component of the second base metal catalyst which reduces and purifies NOx. Further, JP 2008-309013 A describes that an exhaust gas control apparatus having an above configuration facilitates the reduction and purification of NOx by the CO—NO reaction in the second base metal catalyst, and residual CO and HC in the exhaust gas are oxidized and purified in the third base metal catalyst.
Here, in the exhaust gas control apparatus disclosed in JP 2008-309013 A, base metals such as iron (Fe) used as catalyst components have low reduction abilities compared to platinum group metals such as rhodium (Rh), in an atmosphere which has excess oxygen compared to the stoichiometric air-fuel ratio such as a fuel lean atmosphere, or an atmosphere around the stoichiometric air-fuel ratio. Accordingly, when the air-fuel ratio of the exhaust gas is a fuel lean ratio or the stoichiometric air-fuel ratio, NOx contained in the exhaust gas cannot be sufficiently reduced and purified. Hence, for example, when base metals are used as catalyst metals for the NOx purification catalyst, it is in general preferably that the air-fuel ratio of the exhaust gas flowing into the NOx purification catalyst is controlled to a more fuel rich air-fuel ratio than the stoichiometric air-fuel ratio as described in JP 2008-309013 A. However, at such a fuel rich ratio, in other words, in a reducing atmosphere, for example, nitrogen (N2) contained in the exhaust gas is reduced by HC or the like, and ammonia (NH3) may thereby be produced.
Japanese Patent Application Publication No. 2006-289211 (JP 2006-289211 A) and Japanese Patent Application Publication No. 10-085557 (JP 10-085557 A) discloses catalysts and methods for oxidizing and decomposing ammonia contained in the exhaust gas.
For example, an ammonia oxidation catalyst disclosed in Japanese Patent Application Publication No. 2006-289211 (JP 2006-289211 A) prevents the production of nitrous oxide (N2O) from NH3 and oxidizes NH3 to nitric oxide (NO). Accordingly, the ammonia oxidation catalyst disclosed in JP 2006-289211 A does not oxidize and decompose NH3 to nitrogen (N2) and water (H2O) that are harmless. Meanwhile, an exhaust gas control method for purifying an exhaust gas containing ammonia which is disclosed in Japanese Patent Application Publication No. 10-085557 (JP 10-085557 A) is to purify exhaust gases from chemical plants but not to purify an exhaust gas from an internal combustion engine. An exhaust gas control method for purifying an exhaust gas containing ammonia which is disclosed in JP 10-085557 A requires two reactors that are a pre-stage reactor and a post-stage reactor each filled with a catalyst for purifying ammonia. Therefore, in view of cost, installation space, and the like, it is difficult to apply the method disclosed in JP 10-085557 A to an exhaust gas control apparatus for an internal combustion engine.